Their work, published in the latest Systematic Biology journal, follows on from recent work that raised uncertainty about the "single ancestor" theory of the group of flightless birds, known as ratites.

Phillips, of the ANU's Research School of Biology, says ratites are a group of flightless birds that include the Australian emu and cassowary, African ostrich, New Zealand's kiwi and now-extinct moa, rhea from South America and the extinct elephant birds of Madagascar.

The study used molecular dating of the mitochondrial DNA from the moa, which stood 2.5 metres tall and weighed up to 250 kilograms, and found its closest relative to be the tinamous - a group of flighted birds the size of quail, found in South America.

Previously it was thought ratites all shared a common flightless ancestor about 80 million years ago and their worldwide dispersal occurred before the supercontinent of Gondwanaland broke up.

But Phillips says the problem with this theory was that much of the continental break-up occurred well before the proposed common ancestor.

Fewer predators

Their study, which also included DNA sequencing of 22 bird species including flightless and flighted birds, shows ratites became flightless around 65 million years ago, he says.

This coincides with the extinction of dinosaurs in the Cretaceous-Tertiary event.

"Our study suggests that the flighted ancestors of ratites appear to have been ground-feeding birds that ran well," says Phillips.

"In the absence of predators and with abundant food resources on the ground, there is a tendency for birds to evolve larger size and become flightless.

"We see this a lot on islands - dodos for example.

"Larger ground-feeding birds can be more efficient at turning food into growth and reproduction, but with size increase, comes the cost of flight becoming less efficient."

Phillips says the study also throws doubt on the widely held view that the ratite's origins lie in the Gondwana supercontinent that included Africa, South America, Australia, New Zealand, India and the Antarctic.

The study shows the various ratite entities evolved after the break-up of the supercontinent and suggest that the African ostrich - whose fossils have been found in Europe - may have European origins.